In 1988, the World Health Assembly of the World Health Organization
(WHO) resolved to eradicate poliomyelitis by 2000. Since then, the WHO Region of
the Americas and Western Pacific Region have been certified free of polio, and
the European Region is approaching 3 years since the last confirmed case of
polio. Transmission of wild polio-virus types 1 and 3 continues to decline in the other
WHO regions (1). This report summarizes the evidence, obtained through surveillance
for acute flaccid paralysis (AFP), supporting the global interruption of wild poliovirus
type 2 transmission.
Along with achieving and maintaining high routine coverage with oral
poliovirus vaccine (OPV), conducting National Immunization Days* to decrease
poliovirus circulation, and mopping-up vaccination
activities to eliminate remaining
reservoirs§ of poliovirus transmission, one of the main polio eradication strategies is
AFP surveillance. The quality of AFP surveillance is assessed primarily by the nonpolio
AFP rate (target: >1 per 100,000 population aged <15 years), and by the completeness
of specimen collection (target: two adequate stool
specimens¶ from >80% of persons
The last countries to report wild poliovirus type 2 isolates were Afghanistan
and Pakistan in 1997, Nigeria in 1998, and India in 1999
(2). The last known reservoirs of wild poliovirus type 2 transmission occurred in Bihar, Uttar Pradesh, and West
Bengal in northern India. Several type 2 isolates were obtained from this region during
1998--1999. The rapidly declining genetic diversity of the few sustaining type 2 isolate
chains is consistent with the final phase of transmission. The last wild poliovirus type
2 isolated was from a child reported as an AFP case in West Bihar with paralysis onset
in October 1999.
Despite substantially improved AFP surveillance globally since late 1999, no
wild poliovirus type 2 isolates have been reported by any WHO region since late
1999. From 1999 to 2000, the number of AFP cases reported worldwide increased
from 29,583 to 30,436 despite a decrease of confirmed polio cases from 7141 in 1999
to 2824 in 2000. In the South-East Asia Region during 1999--2000, the overall
nonpolio AFP rate increased from 1.6 to 1.7 per 100,000 population aged <15 years, and
the rate of adequate stool collection increased from 71% to 81%, respectively. In
the Eastern Mediterranean Region, the overall nonpolio AFP rate increased from 1.1 to
1.4 and the rate of adequate stool collection remained at 67%. In the African Region
1999--2000, the overall nonpolio AFP rate increased from 0.8 to 1.3; however, the
rate of adequate stool collection (53%) remained below the 2000 target level.
Surveillance remains suboptimal in the major reservoir countries of Angola, Democratic Republic
of Congo, Ethiopia, and Nigeria.
AFP surveillance comprises a global network of seven specialized, 15
reference, and 126 national WHO-accredited laboratories. The network processed 48,370
stool specimens in 1999 and approximately 50,000 in 2000. During 1999--2000,
1423 isolates were wild poliovirus type 1 (989 in 1999 and 434 in 2000); 11 were
wild poliovirus type 2 (11 in 1999 [from India] and zero in 2000); 1127 were wild
poliovirus type 3 (894 in 1999 and 233 in 2000), and 23 were wild poliovirus types 1 and 3
mixed isolates (16 in 1999 and seven in 2000) (Table 1).
Reported by: Vaccines and Biologicals Dept, World Health Organization,
Geneva, Switzerland. Respiratory and Enteric Viruses Br, Div of Viral and Rickettsial
Diseases, National Center for Infectious Diseases; Vaccine Preventable Disease Eradication
Div, National Immunization Program, CDC.
The apparent elimination of wild poliovirus type 2 represents
a milestone for the global polio eradication initiative and an indication that the
current strategies can eradicate poliovirus types 1 and 3. Since late 1999, the global
polio laboratory network has processed tens of thousands of stool specimens,
including those from countries at high risk for undetected poliovirus circulation. All
polioviruses type 2 isolated since October 1999 have been vaccine derived, and the
declining genetic diversity of the last wild isolates from India is consistent with the final
phase of transmission.
Before the advent of the polio vaccine, wild poliovirus type 2 had
worldwide distribution. As the vaccine was introduced, particularly in temperate climates,
wild poliovirus type 2 transmission disappeared quickly. Transmission continued
in countries with high population density and poor sanitation, but disappeared
more quickly than other polio-virus types as vaccination rates improved. The
high immunogenicity of type 2 polio-viruses in OPV and the efficient spread of the
vaccine-derived strain from vaccinated persons to close contacts may be important factors
in its earlier disappearance.
Although the likelihood of undetected transmission decreases with time,
evidence of interruption of type 2 transmission is reinforced with continued improvement in
AFP surveillance, particularly in Africa, where the nonpolio AFP rate and rate of
timely specimen collection remain inadequate in some high-risk countries. In addition,
the increased laboratory workload generated by improving stool collection rates must
be met with additional human and financial resources to maintain the quality
and timeliness of specimen processing.
Although wild polioviruses types 1 and 3 have been more difficult to control
than type 2, the experience in the Americas, Western Pacific, and Europe underscores
the feasibility of global eradication of all wild poliovirus serotypes.
- CDC. Progress toward global poliomyelitis eradication, 1999. MMWR 2000;49:349--54.
- CDC. Progress toward the global interruption of wild poliovirus type 2 transmission,
1999. MMWR 1999;48:736--9.
* Nationwide mass campaigns during a short period (days to weeks), in which two
doses of OPV are administered to all children in the target age group (usually aged <5
years), regardless of vaccination history, with an interval of 4--6 weeks between doses.
Focal mass campaigns in high-risk areas during a short period (days to weeks) in
which two OPV doses are administered to all children in the target age groups, regardless
of vaccination history, with an interval of 4--6 weeks between doses.
§ Countries where polio is endemic that have large populations and that may
export poliovirus to neighboring countries and elsewhere.
¶ Two stool specimens, collected 24 to 48 hours apart within 14 days of paralysis
onset, that arrive in the laboratory in good condition.
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